1. Field of the Invention
The present invention relates to proximity sensors. More particularly, the present invention relates to "closed field", capacitive proximity sensors for measuring force, displacement and pressure.
2. Discussion of Background
Many techniques currently exist for detecting the presence of solid objects or liquids in specified locations, for measuring small displacements or forces acting upon them, and for measuring absolute or differential pressures in gases or liquids. For compatibility with modern display and control devices, nearly all transducers now manufactured to sense these quantities are electronic or electrical in nature.
The majority of force transducers employ strain gauges: arrays of matched resistors, typically made of metallic foil, and so configured that mechanical strain causes the resistance of certain of these resistors to increase relative to that of others. Typically, a strain gauge is attached to a solid body of elastic material, such as steel, which bends or otherwise deforms in response to the applied force, so that strain in the support is transferred to the gauge and produces the electrical output.
Similarly, gas pressure and mechanical displacement may be measured by means of thinner, elastic elements, such as diaphragms, to which strain gauges are attached. Flexing in response to gas pressure, or to a change in mechanical shape resulting from relative movement between objects bridged by the elastic element, transfers strain to the gauge and produces the output.
Strain gauges have represented the technology of choice for many years, since they are easily made, usually inexpensive, and give outputs which are approximately linear with respect to the amount of strain which is present. However, the output of such a device is typically a very small fraction of the applied driving voltage, is very sensitive to driving-voltage fluctuations, and appears at a fairly high impedance of several hundred to a few thousand ohms; these disadvantages make strain gauges difficult to use in remote or electrically-noisy locations.
Several other sensing methods also exist, and are widely used in detecting the presence of objects or, in some cases, in measuring mechanical displacement. Typically, a steady or fluctuating electromagnetic field is set up and its interaction with the immediate environment, including nearby materials, is observed. While most methods are limited to sensing materials with high electrical conductivity, magnetic permeability or optical reflectance, electric fields at moderate to high frequencies can be used to sense a wide variety of materials through the changes in the magnitude of displacement current flowing into or out of the sensing field, resulting from varying capacitance. Such a sensing field is usually projected by a flat metal plate which faces the object or location to be sensed. For instance, an array of small metal-plated areas on the back of a sheet of glass, and connected to appropriate circuitry, is often used as a convenient, long-lasting and easily-cleaned control panel for a microwave oven or other kitchen appliance.
Typically, capacitive sensing uses an "open" sensing field: one which provides no inherent return path for the displacement current flowing between the sensing field and the object to be detected. For proper functioning, therefore, an adequate return path must be provided by electrically-conductive materials or distributed capacitance in the environment. Operation is thus quite dependent on factors over which the user may have little or no control. High levels of electrical or magnetic noise may also enter the open sensing field from the environment, causing interference and "false positive" readings.
Were it not for the weakness resulting from the open nature of the sensing field, capacitive proximity sensing would be readily adaptable to the vast majority of position, proximity, strain, displacement and pressure sensing applications.